In the manufacturing of various products, it is necessary to apply a layer of protective material on a surface, which must be removed after a specified manufacturing step has been concluded. An example of such a process is the so-called "masking", where a pattern is created on a surface using a layer of protective material illuminated through a mask, and the surface is then treated with a developer which removes material from the unmasked portions of the surface, therefore leaving a predetermined pattern. The surface is then treated by ion implantation or by etching agents, which introduce the imprinted species into the unmasked portions of the surface, or remove material from unmasked portions. Once these processes are completed, the role of the protecting mask ends and it must be removed. The process is conventional and well known in the art, and is described, e.g., in U.S. Pat. No. 5,114,834.
Two main photoresist stripping methods exist in the modern VLSI/ULSI (Very/Ultra Large Scale Integration) circuits industry:
1) Wet stripping, which uses acids or organic solvents; PA1 2) Dry stripping, which uses plasma, O.sub.3, O.sub.3 /N.sub.2 O or UV/O.sub.3 -based stripping. PA1 1) The UV-Laser (EXCIMER or SOLID STATE) should have a high average power P.sub.av.gtoreq.100W; PA1 2) Its pulse frequency f should be low (f.about.50 Hz); PA1 3) Its pulse energy E should be sufficiently high (E.gtoreq.1 J); and PA1 4) Its pulse should have a controlled extended duration (up to 200 ns and more). PA1 F.sup.ph1.sub.opt, F.sup.ph2.sub.opt, . . . , F.sup.phi.sub.opt, . . . PA1 1) The laser source must have a high average power; PA1 3) The working fluence must have optimal values at the appropriate optimal pulse extension, viz. maximum throughput when all other parameters are the same; PA1 4) The pulse frequency (pulse repetition rate) must be fairly low for the balanced performance of laser radiation impact and gas flow in a process chamber. In other words, it is necessary to permit a fresh portion of process gas to flow on the beam footprint (by "footprint" is meant the area on which the beam impinges on the surface being treated) after every pulse, and this limits the pulse frequency to relatively low values, e.g., in the order of 50-100 Hz.
Both methods are problematic and far from being complete, especially when taking into consideration the future miniaturization in the VLSI/ULSI industry. The current technology is capable of dealing with devices having feature sizes of about 0.5 m, but before the end of the century, the expectation is that the workable size of the devices is to be reduced to 0.25 m. The anticipated size change requires considerable changes in the manufacturing technology, particularly in the stripping stage. The prior art photoresist stripping techniques described above will be unsuitable for future devices, as will be explained hereinafter.
Utilizing only the wet stripping method is not a perfect solution, as it cannot completely strip photoresist after tough processes that change the chemical and physical properties of the photoresist in such a way that makes its removal very difficult. Such processes include, e.g., High Dose Implantation (HDI), reactive Ion Etching (RIE), deep UV curing and high temperatures post-bake. After HDI or RIE, the side walls of the implanted patterns or of the etched walls are the most difficult to remove.
In addition, the wet method has some other problems: the strength of the stripping solution changes with time, the accumulated contamination in the solution can be a source of particles which adversely affect the performance of the wafer, the corrosive and toxic content of stripping chemicals imposes high handling and disposal costs, and liquid phase surface tension and mass transport tend to make photoresist removal uneven and difficult.
The dry method also suffers from some major drawbacks, especially from metallic and particulate contamination, damage due to plasma-charges, currents, electric fields and plasma-induced UV radiation, as well as temperature-induced damage, and, especially, incomplete removal. During various fabrication stages, as discussed above, the photoresist suffers from chemical and physical changes which harden it, and this makes the stripping processes of the prior art extremely difficult to carry out. Usually, a plurality of sequential steps, involving wet and dry processes, are needed to completely remove the photoresist.
The art has addressed this problem in many ways, and commercial photoresist dry removal apparatus is available, which uses different technologies. For instance, UV ashers are sold, e.g. by Hitachi, Japan (UA-3150A), dry chemical ashers are also available, e.g., by Fusion Semiconductor Systems, U.S.A., which utilize nitrous oxide and ozone to remove the photoresist by chemical ashing, microwave plasma ashing is also effected, e.g., as in the UNA-200 Asher (ULVAC Japan Ltd.). Also, plasma photoresist removal is employed and is commercially available, e.g., as in the Aspen apparatus (Mattson Technology, U.S.A.), and in the AURA 200 (GASONICS IPC, U.S.A.).
More recently, photoresist removal has been achieved by ablation, using laser UV radiation, in an oxidizing environment, as described in U.S. Pat. No. 5,114,834. The ablation process is caused by strong absorption of the laser pulse energy by the photoresist. The process is a localized ejection of the photoresist layer to the ambient gas, associated with a blast wave due to chemical bonds breaking in the photoresist and instant heating. The partly gasified and partly fragmented photoresist is blown upwards away from the surface, and instantly heats the ambient gas. Fast combustion flash of the ablation products occurs, after each laser pulse, due to the photochemical reaction of the UV laser radiation and the process gases, which may also be due to the blast wave. The main essence of the process is laser ablation with combustion flash of the ablated photoresist, which occurs in a reactive gas flowing through an irradiation zone. The combination of laser radiation and fast combustion provides instantaneous lowering of the ablation threshold of hard parts of the photoresist (side walls). The combusted ablation products are then removed by vacuum suction, or by gas sweeping, leaving a completely clean surface.
While U.S. Pat. No. 5,114,834 provides an important novel process, it still does not provide a high throughput, which is industrially convenient, viz. an industrially acceptable number of wafers that can be stripped during a given time. The laser stripping throughput is determined by the stripping rate or by the number of laser pulses needed for providing complete stripping of a unit area of the photoresist per unit of time.
While reference will be made throughout this specification to the ablation of photoresist from semiconductor wafers, this will be done for the sake of simplicity, and because it represents a well known and widely approached problem. It should be understood, however, that the invention described hereinafter is by no means limited to the stripping of photoresist from wafers, but it applies, mutatis mutandis, to many other applications, such as stripping and cleaning of photoresist from Flat Panel Displays (FPD) or removal of residues from different objects, such as lenses, semiconductor wafers, or photo-masks.
The aforementioned U.S. Pat. No. 5,114,834 defines the process window of laser stripping, and indicates that there are certain energy fluence levels of the laser pulse which may damage the wafer being treated. So far, however, the art has failed to provide a method which conveniently permits to utilize the energy of an excimer laser in a way that allows to increase the fluence damage threshold defined in U.S. Pat. No. 5,114,834, without incurring the risk of damaging the surface of the object being treated. The types of damage due to laser energy include thermal damages, such as ripples due in particular to difference in expansion coefficients, e.g., SiO.sub.2 /Si (implanted) and TiN/Al interfaces and related to the fatigue phenomena, aluminum or silicon melting, as well as radiation (ionization) damages, e.g., slight color changes due to small changes in the crystalline structure at SiO.sub.2 /Si interface (implanted).
WO 97/17164 (PCT/IL/00139), the entire content of which is incorporated herein by reference, discloses a method of damage-free laser surface treatment by extending a laser pulse in time and supplying the same pulse energy to a treated surface during a longer period of time. The pulse extension is carried out by optical means, viz. by means of a Passive Optical Pulse Extender, hereinafter "POPE".
Continuous increasing dimensions of wafers for Next Generation UNVLSI circuits (from 6" to 12", last decade) and the numerous difficulties linked with miniaturization of features of wafers (new photoresists, metals, oxides, higher demands to cleaning, etc.,) complicate severe stripping and cleaning processes, affecting both their yield and throughput.
For high yield throughput laser removal of foreign material from semiconductor wafers, the following conditions must be fulfilled:
Any high average power industrial UV-Laser has a high pulse frequency f.gtoreq.200-250 Hz, low pulse energy E.ltoreq.0.5 J and constant pulse length. A Passive Optical Pulse Extender (POPE) does increase the duration of laser pulse while not being able to change (regulate) it, since optical delays are fixed for any given case. Therefore, a combination of any commercial high average power UV-Laser+POPE does not possess the second, third and fourth of the above-mentioned features needed for providing high (optimal) throughput.
It follows from the aforementioned WO 97/17164 that, for processing in the range of optimal fluence value F.sub.opt, one needs to provide an appropriate fluence process window. This means that it is necessary to provide a definite pulse extension, thereby decreasing the thermal load on a substrate to prevent its damage.
A variable pulse extension permits to find the approximate optimal fluence values for different types of photoresist:
where F.sup.phi.sub.opt is the fluence for the i-th type of photoresist. Each value of F.sup.phi.sub.opt requires its pulse extension. The implementation of a number of pulse extensions needs the continuous changing of time intervals between subpulses produced in optical pulse extenders for pulse extension (see the cited WO 97/17164). Thus, changing pulse extension would provide both a damage-free and an optimal Multi-Laser Combustion (MLC) surface treatment. However, the optical pulse extender has constant optical delays, and therefore cannot provide a controlled pulse extension, as well as an optimal MLC surface treatment.
As noted in the aforesaid WO 97/17164, the fluences required for laser removal of foreign materials can be so high that the laser pulse duration must be extended many times to avoid damage of the treated surface. However, the combination of high power laser with optical pulse extension is not sufficient in many cases, because it cannot provide a sufficiently low pulse frequency and controlled pulse extension, and, whenever a large pulse extension is required, very high losses of pulse energy occur in the passage of the pulse through the elements of the optical extender. Control of the pulse extension is required, because it is necessary to provide in each case an appropriate fluence process window to avoid thermal damage of the treated substrate. Different types of photoresists have different optimal fluence values and each of these requires a different pulse extension. Therefore, controlled pulse extension provides a MLC surface treatment that has an optimal throughput and avoids substrate damage.
A stabilized operation of industrial High Power Lasers (HPL), in particular, HP UV-Excimer Lasers, requires that the pulse energy be relatively low and the pulse frequency high, since the very intensive pumping, which is required to obtain high pulse energy, inevitably causes some instabilities in laser active medium. By "laser active medium" is meant a gas or a solid which, after excitement by discharge or by flash lamp, is capable of generating a laser beam, if a resonator is provided. On the other hand, both low pulse energy and high pulse frequency lead to a lowering of throughput, due to their unfavorable influence on the efficiency of the optical line.
Low pulse energy can provide high optimal values of working fluence only on small areas of laser beam impact. It decreases the efficiency of fluence homogenization on a wafer. In turn, high pulse frequency is not acceptable for dry laser chemical stripping, as hereinbefore explained, and requires a fitting dividing by optical means. As has been said, reduction of the pulse frequency by optical means also causes a significant laser energy loss. Therefore, both these factors sharply deteriorate the efficiency of the optical line.
The above-mentioned causes of decrease of the laser stripping throughput prevent the use of any industrial High Power Excimer Laser for an efficient laser photoresist stripping technology.
It is a purpose of this invention to provide a method and an apparatus for the generation of laser beams, that produce an improvement in surface treatments, particularly in the removal of foreign materials from substrate surfaces, when carried out by irradiation with said laser beams.
It is another purpose of this invention to render possible and efficient the application of High Power Laser Multi-Head systems for photoresist stripping operations.
It is further purpose of this invention to provide a a method and an apparatus for the generation of laser beams that permit to effect MLC surface treatments with high throughput and without damages to the treated surface at a high degree of cleanliness (high yield).
It is a still further purpose of this invention to provide an apparatus for MLC surface treatment with high throughput and without damages to the treated surface at a high degree of cleanliness.
Other purposes and advantages of this invention will appear as the description proceeds.